The present invention relates to a so-called inverted dryer group provided with a single-wire draw in a multi-cylinder dryer in a paper machine, which dryer group comprises steam-heated drying cylinders arranged in a lower row. A drying wire of the cylinder group presses the web to be dried against, and into direct contact with, the steam-heated drying cylinders. In the dryer group, reversing cylinders or rolls of the dryer group are placed above gaps formed between the drying cylinders. The reversing cylinders or rolls are arranged inside a loop of the drying wire. The web to be dried is placed at the side of the outside curve on turning sectors of the reversing cylinders or rolls. A support wire is arranged to reach contact with the reversing cylinders or rolls over a substantially large sector. The support wire is guided by guide rolls placed in the gaps between the reversing cylinders as well as by other necessary rolls. The web is pressed against the drying wire on the turning sectors by means of the tension of the support wire. The present invention also relates to a method to prevent shrinkage of a paper web by utilizing the inverted dryer group in a dryer section of a paper machine.
In the prior art, in multi-cylinder dryers in paper machines, twin-wire draw and/or single-wire draw is/are employed. In the case of a twin-wire draw, the groups of drying cylinders include two wires which press the web against heated cylinder faces, i.e., one wire from above and the other one from below. Between the rows of cylinders, which are usually horizontal rows, the web has free and unsupported draws. The free draws are susceptible of fluttering which may result in web breaks. For this reason, in recent years, increasing use has been made of the single-wire draw in which there is just one drying wire in each group of drying cylinders. The web runs on support of the single drying wire through the entire dryer group so that the drying wire presses the web against the heated cylinder faces of the drying cylinders and the web remains at the side of the outside curve on the reversing cylinders placed between the drying cylinders. Thus, in a single-wire draw, the drying cylinders are placed outside the wire loop, and the reversing cylinders are arranged inside the loop of the drying wire.
In prior art normal dryer groups having a single-wire draw, the heated drying cylinders are placed in an upper row, and the reversing cylinders are placed in a lower row. The upper and lower rows are usually arranged to be horizontal and parallel to one another. In the assignee's Finnish Patent No. 54,627 (corresponding to U.S. Pat. No. 4,202,113, the specification of which is hereby incorporated by reference herein), an arrangement is described in which normal single-wire groups, mentioned above, and so-called inverted single-wire groups are arranged one after the other. In the inverted groups, the heated drying cylinders are arranged in the lower row and the reversing suction cylinders or rolls are arranged in the upper row in order to achieve the principal object, i.e., drying the web symmetrically from both of its sides.
In the prior art, reference is also made to International Patent Applications No. WO 88/06204 and WO 88/06205 of Messrs. Beloit Corp. which describe a dryer section consisting of normal and inverted cylinder groups.
In the following description, the terms "normal (dryer) group" and "inverted (dryer) group" are used and intended to designate cylinder groups having a single-wire draw similar to those described above.
Further, with respect to the prior art related to the present invention, reference is made to the assignee's Finnish Patent No. 62,693 (corresponding to U.S. Pat. No. 4,481,723, the specification of which is hereby incorporated by reference herein), which describes an arrangement wherein a lower support fabric is used in a normal group having a single-wire draw, together with pressurization of the pockets defined by them.
In prior art inverted dryer groups, cylinders or rolls which have perforated mantles with a grooved outside face and whose interior communicates with a source of negative pressure are used as reversing cylinders. By the effect of this negative pressure, the web placed on the drying wire at the side of the outside curve is held in position, and it is ensured that the web does not stretch or even become completely separated from the outside face of the drying wire in the curves. The suction cylinders are quite expensive components, and they require suction energy, ever more suction energy being consumed when the speed of the paper machine becomes higher.